Various climbing assist devices are used to assist personnel in climbing ladders over extended distances and to assure their safety in the event of a fall. Oil derricks and work-over rigs present special problems in protecting against injury resulting from falls not only by virtue of the distance over which personnel are required to climb, oftentimes with the additional weight of tools and accessories, but as a result of adverse weather conditions and other natural hazards associated with that work. Typically, apparatus which is presently in use includes a counterweight to assist personnel in climbing to the desired height together with means to control their descent or escape particularly if one should accidentally slip or fall from the ladder. It has been proposed in the past to employ hydraulic cylinders positioned alongside of the derrick or climbing structures to serve as a retarding means in the event of a fall. However, these cylinders can be easily damaged and rendered inoperable, for example, in the event of a dent in the cylinder which will prevent it from performing properly. Hydraulic cylinders further are not capable of controlling the descent of personnel at a constant speed irrespective of the weight of the personnel. In other words, unless the descent control apparatus is capable of establishing a controlled rate of descent independently of the weight of the person, severe limitations are placed on its use by different weight persons.
Representative of approaches taken in the past are those disclosed in U.S. Pat. No. 4,252,214 to Miller which employs a hydraulic cylinder as a means of controlling descent. U.S. Pat. No. 2,948,348 to Bott as well as U.S. Pat. No. 2,538,904 to Herod disclose derrick or ladder safety devices in which counterbalancing mechanisms are employed in combination with descent control apparatus. Other representative patents are U.S. Pat. Nos. 1,571,096 to L. G. Rambin et al; 1,849,725 to D. E. Quick; 2,670,890 to S. P. Hodge; and 3,826,335 to M. F. Allen.